1. Field of the Invention
The invention relates to a method of extracting motion errors of a carrier bearing a coherent imaging radar system from radar raw data in which over a predetermined period of time consecutive azimuth spectra are continuously formed, for calculating a frequency shift over the entire azimuth spectrum (S(f,i)) a correlation (K.sub.O (f,i)=S(f,i)xS(f,i-1) is carried out with S(f,i) as azimuth spectrum at the instant i, with S( f ,i-1) as azimuth spectrum at the instant (i-1) and with x as correlation), thereafter frequency shift values are processed and the displacement in line of sight direction of the antenna and the forward velocity are calculated. The invention also relates to an apparatus f or carrying out this method comprising a means for calculating the azimuth spectrum (S(f,i)) for continuously forming consecutive azimuth spectra from radar raw data and a means for forming a correlation (K.sub.0 (f,i)) between respectively two azimuth spectra (S(f,i); S(f,i-1)) formed immediately consecutively in time.
2. Description of the Prior Art
Coherent imaging radar systems are installed in a carrier, for example an aircraft, a missile or a helicopter and the like. Normally, due to the conditions relating to such carriers and due to the influences of the environment, for example due to turbulences, a carrier cannot keep to a predetermined flight path. On the contrary, the carrier deviates from the desired reference flight path, and this is referred to as motion errors. With coherent imaging radar systems motion errors lying in the order of magnitude of the wavelength of the radar transmission signal cause distortions by which the quality of an image is very considerably impaired. The quality of an image is assessed by its resolution, contrast and the geometric distortions. In imaging, to obtain a high resolution, high contrast and low geometric distortions, the received raw data must be corrected before the processing or generating of the image. Such a correction can be carried out in real time or off-line. A real-time correction takes place during the reception of the backscatter signals with the aid of digital or analog actuators. An off-line correction is carried out on the ground with the aid of computing programs after storage of the raw data.
The processing or generation of an image cannot be carried out until after a correction, which is referred to as motion compensation. For generating an image a correlation is carried out between the raw data and the theoretical phase history to be expected and it is not until after such a correlation that a high-resolution two-dimensional image is obtained.
Here, the motion errors of the carrier must be determined very accurately to enable the raw data to be corrected. For this purpose, the following movements of the carrier must be determined, i.e. the forward velocity and the deviation in the viewing direction of the antenna.
All known real-time motion compensation systems are dependent on on-board inertial navigation systems (INS) and other navigation systems, such as GPS (Global Positioning System). All motion extraction methods are methods in which the motion errors of the carrier can be extracted from the radar raw data. For example, for this purpose the so-called autofocus method and also the so-called shift method known from DE 39 22 428 C2 may be used; with the shift method referred to above considerably better results can be achieved.
However, all the known motion compensation systems have various disadvantages. Due to the high demands made on the motion data as regards accuracy, bandwidth and time stability, for imaging radar systems in which a motion compensation is carried out inertial navigation systems made for example in special production must be used. This also however results in very high acquisition, installation and maintenance costs.
When using motion compensation systems operating with GPS receivers support by a ground station is indispensable. This however firstly makes the operating costs very high and secondly there are considerable restrictions in the choice of the use territory. However, without support by a ground station the GPS motion data absolutely essential to motion compensation are not accurate enough.
Although the autofocus methods are not dependent on inertial navigation systems, they nevertheless be carried out in real time because of the high computing expenditure. In addition, autofocus methods do not have a high bandwidth nor a high accuracy and consequently in particular the motion errors due to wind gusts cannot be corrected. For this reason, autofocus methods are thus normally only used for estimating the forward velocity of a carrier.
In motion compensation systems operating with the so-called shift method according to DE 39 22 438 C2, the shift between two azimuth spectra consecutive in time is evaluated. The shift here is determined by the position of the maximum of the correlation of the two azimuth spectra not however taking account of gaps in the correlation. Such gaps occur in the event of a pronounced time variation of the backscatter ratio component of the azimuth spectrum, for example on appearance of a pronounced point target in one of the azimuth spectra which are being correlated.